1. Field of the Invention
The present invention relates to polymeric compounds with nonlinear optical properties, and more particularly, to organic dye molecular materials and nonlinear optical polymer compounds containing chromophores.
2. Description of the Related Art
Recently, the development of high-speed, high-capacity data transmission devices has increased the need for materials that exhibit nonlinear optical properties and research in this field is being actively performed (Ind. Eng. Chem. Res. 1999, 38, 8-33). These nonlinear optical materials are roughly categorized into inorganic materials such as LiNbO3, KHP, quartz and the like, which have been widely used, and organic materials which become more interesting in the field in recent years along with semiconductor materials. Organic materials are advantageous over inorganic materials in terms of their synthesis and processing procedures and that their physical properties including the processing temperature, refractive index, nonlinear optical coefficient, absorption wavelength and the like can be adjusted according to the requirements. This is the reason why research on the organic material becomes increasing in the field (U.S. Pat. Nos. 5,496,899 and 6,229,047).
Organic materials are classified into crystalline molecules and polymeric substances based on their structure. In the early stage of research, many researchers in a variety of fields were very interested in the crystalline molecule due to its large optical coefficient. However, the crystalline molecule is limited by its unstable nature and difficulties in the crystal growing and processing. To address these limitations, approaches to dispersing organic molecules in polymeric media to develop new materials have been actively made in recent years (Chem. Mater. 1999, 11, 1966-1968). The molecular mobility in a polymeric medium reduces thermal stability and molecular aggregation causes significant optical losses, thereby limiting use of the material.
Meanwhile, to develop materials with desirable processing properties of polymers by incorporating nonlinear organic molecules into polymers is greatly acceptable. Nonlinear polymers are classified into main chains and side chains depending on the way of molecular coupling. Recently, highly ordered macromolecules coupled to be chemically stable, called xe2x80x9cdendrimerxe2x80x9d, have been developed and many approaches have been made to increase their applications (U.S. Pat. Nos. 5,659,010, 5,496,899, and 6,001,958).
In another aspect of research, some researchers are involved in developing new materials based on covalently-bonded film formation technique improved from the Languir-Blodegette (LB) film formation technique. Although it is difficult to obtain a thin film having an appropriate thickness for use with this method, the resulting thin film has a structure highly ordered to give a very large nonlinear coefficient.
For main chain polymers, nonlinear molecular chromophores are directly used as molecules to be polymerized so that the synthesis of the main chain polymer is difficult and the resulting polymer has poor three-dimensional orientation efficiency during a poling process. For side chain polymers, chromophores are grafted to the core as side chains. In this case, although it is difficult to chemically react the polymer, the selection of core polymer is flexible and a variety of chromophores can be incorporated. The ordered arrangement of chromophores on the polymer chain as its side chains is also advantageous.
Dendrimers are sterically highly symmetric and exhibit different characteristics according to their functional end groups incorporated. Also, appropriate designing of a dendrimer molecular structure can separate individual chromophores. When excess chromophores over a predetermined amount are incorporated into a polymeric material, aggregation occurs due to chromophore-chromophore electrostatic interactions, thereby resulting in a low nonlinearity and optical scattering loss due to micro domain. Recent studies evidently show that these problems can be eliminated by using dendrimers (Appl. Phys. Lett. 2000, 77(24), 3881-3883).
Basically, the development of highly nonlinear materials needs highly nonlinear dye molecules. According to recent research results, a nonlinear molecule needs a structure capable of partial electron polarization and a large dipolar moment to provide a large nonlinearity. It is advantageous that the nonlinear molecule has strong electron donor and electron acceptor end groups. In addition, the nonlinear molecule should be extended by conjugated double or triple bonds to allow the electrons, i.e., the xcfx80-electrons, present between the end groups to move freely. It has been also founded that the nonlinearity effect is increased when such conjugated linkages are stably present on the same plane. Some materials with extended xcfx80-linkages to increase their nonlinearity have been reported. As the number of xcfx80-linkages increases, the visible light absorption region of the material where electron transition absorption occurs is gradually shifted to a long-wavelength region. This directly affects on the optical loss by absorption, thereby limiting its use as an optical device material (J. Am. Chem. Soc. 1999, 121, 472-473; Chem. Mater. 1999, 11, 1966-1968). As an example, a wavelength converter or an optical amplifier is often used as a light source of a wavelength of about 600-800 nm. Thus, it is necessary to develop materials that are transparent in this wavelength range for the optical communication device.
Meanwhile, the nonlinearity is directly proportional to the chromophore density in a unit volume. As described above, a high chromophore density over a predetermined range results in a reduced nonlinearly. Therefore, there is a need to increase the chromophore density in a polymeric material without this adverse effect.
To solve the above-described problems, it is a first objective of the present invention to provide an organic dye molecular material suitable for use in the formulation of an optical polymeric compound having a high nonlinearity.
It is a second objective of the present invention to provide a nonlinear optical polymeric compound for use as an optical device material that comprises molecules capable of absorbing a short-wavelength light, has a high dye molecule density, and is spatially highly ordered to provide a high nonlinearity.
To achieve the first objective of the present invention, there is provided an organic dye molecular material having the following formula: 
where X1 is hydrocarbon, oxygen, sulfur, nitrogen, ester (CO2), or amide (CONR1), where R1 is an alky or phenyl group having 1 to 6 carbon atoms, D is an organic chromophore molecule, and n is an integer from 1 to 10.
To achieve the first objective of the present invention, there is provided another organic dye molecular material the following formula: 
where D is an organic chromophore molecule, and n is an integer from 1 to 10.
In formulae (1) and (2) above, the organic chromophore molecule D may be any well-known organic chromophore. For example, the organic chromophore molecule D may have a structure selected from structures (A-1), (A-2) and (A-3) in the following formula (3) in which each chromophore molecule is shown as D-OH: 
where R and Rxe2x80x2 are each independently alkyl or phenyl groups having 1 to 10 carbon atoms, A1 is carbon or nitrogen, X2 is NO2, a sulfonyl-substituted or unsubstituted alkyl group having 1 to 10 carbon atoms, CN, xe2x80x94C(CN)xe2x95x90C(CN)2, an ester group, a carbonyl group, a halogen element, or a haloalkyl group, and n is an integer from 1 to 11.
In conjunction with formula (3), typical examples of the organic chromophore molecule having the structure (A-1) include Disperse Red 1 (DR1) series where Rxe2x95x90CH3CH2, A1=N, X2xe2x95x90NO2, and n=2, dialkylaminophenyl nitrostilbene (DANS) series where Rxe2x95x90CH3CH2, A1=CH, X2xe2x95x90NO2, and n=2, DASS series where Rxe2x95x90CH3 or CH3CH2, A1=CH, X2xe2x95x90SO2, and n=2, and the like. A typical example of the organic chromophore molecule having the structure (A-2) includes AIDC derivatives where Rxe2x95x90CH3CH2, CH3, or phenyl, X2xe2x95x90CN, Rxe2x80x2xe2x95x90CH3, and n=2.
To achieve the second objective of the present invention, there is provided a nonlinear optical polymeric compound containing polyimide repeating units to which the organic dye molecular material having the formula (1) or (2) above is coupled.
It is preferable that the nonlinear optical polymeric compound according to the present invention comprises a polyimide main chain having formula (4) or (5) below. 
The nonlinear optical polymeric compound may have a linear homopolyimide backbone. Alternatively, the nonlinear optical polymeric compound can have a copolymer backbone of polyimide repeating units and other repeating units.
The nonlinear optical polymeric compound preferably have a number average molecular weight of 5,000-500,000.
In the nonlinear optical polymer compound according to the present invention, the polyimide repeating unit preferably has the following formula: 
where A and B are each independently fluorocarbon-substituted or unsubstituted hydrocarbons having 1 to 4 carbon atoms, oxygen, nitrogen, or sulfur, and m is in the range of 0.01 to 1 as the ratio of the polyimide repeating units to all the repeating units of the optical polymeric compound.
More preferably, the polyimide repeating unit has the following formula: 
In the nonlinear optical polymer compound according to the present invention, the amount of the organic chromophore molecule D being coupled to the polyimide repeating unit may be flexibly adjusted. Also, the organic chromophore molecule may be composed of one kind of organic chromophore molecules or different kinds of organic chromophore molecules in a predetermined ratio. Preferably, the polyimide repeating unit contains 10-60% by weight of the organic chromophore molecule D.
According to the present invention, commonly used organic chromophore molecules are coupled into the structure of formula (1) or (2) to give organic dye materials having the new structures. When the organic dye materials according to the present invention are incorporated into optical polymeric compounds, the amount of dye per unit volume of the polymer compound can be highly increased.
The nonlinear optical polymeric compound according to the present invention meets the optical material requirements as the molecular aggregation caused by electrostatic interactions between dye molecules does not occur in the compound at high dye densities. In addition, the optical polymeric compound according to the present invention is suitable for use in the manufacture of an easy-to-manufacture device with excellent physical and chemical stabilities as well as nonlinear optical effects.